Wireless communication systems can employ different types of transceivers. Traditional heterodyne transceivers, for example, employ most of the required gain at an Intermediate-Frequency (IF), between the Radio Frequency (RF) and baseband. While very attractive for high-performance applications, heterodyne transceivers require IF components, which cannot be integrated on-chip, thereby increasing the cost. Direct-conversion transceivers, as the name implies, convert directly between RF and baseband, and hence have become popular for Integrated Circuits (ICs) to be used in low-cost equipment.
It is well known in the art that the bandwidth of wireless transmissions can be increased by transmitting data as complex components, e.g., general electronic signals can be represented as complex quantities mathematically, and that this can be viewed as using both phases of a carrier signal. Thus, it is generally accepted that a signal can have a real component synonymous with the real signal impressed upon the cosine carrier and identical to the in-phase (I) signal, and an imaginary component synonymous with the real signal impressed upon the sine carrier and identical to the quadrature (Q) signal. When not on a carrier, which is referred to as baseband, these I and Q signals exists as real signals in two channels commonly called I and Q, respectively. Because direct-conversion transceivers must realize most of the gain required for transmission and for reception at baseband, it becomes problematic to realize direct-conversion transceivers with adequate gain balance between the I and Q channels for some applications. Even heterodyne-conversion transceivers can be difficult in such situations; their baseband elements may not achieve adequate I-Q gain balance, especially at low supply voltages.
Bandwidth-efficient digital modulations, e.g., M-QAM and M-PSK, employ information on both in-phase and quadrature components of the carrier. As the number of constellation points M becomes large, the constellation points become close together; distortions of the complex amplitude eventually become the limiting factor in reducing symbol errors. The trend toward lower supply voltages further exacerbates this problem. While the performance of digital ICs eventually suffer at supply voltages below approximately 1V, poorer control of circuit parameters makes the performance of analog ICs difficult with power supplies below about 2.5V. Thus, calibration in order to balance the gains provided in the I and Q channels for both transmit and receive becomes critical for ICs designed to operate on low-voltage supplies.
While injection of DC calibration signals to calibrate the baseband transmit and receive gains is possible, the inclusion of this function presents a layout difficulty for critical circuitry. Furthermore, such signals would not provide calibration of the effective conversion gains of the RF up-conversion and down-conversion mixer elements.
The present invention is designed to improve the quality of transceivers operating with low voltage power supplies so as to support higher signaling alphabets. The calibration technique must be comprehensive in the sense of calibrating the transmit and receive chains independently and also including all relevant gains in the calibration process. Finally, the calibration process should place less design stress on the circuitry than would be required using commonly known alternate calibration approaches such as the ones described below.